PROF. OWEN" OX AEGILLORNIS LOXGIPENNIS. 127 



the port of determination is the linear ridge above mentioned, viz. 

 the " ancono-deltoid ridge." It varies, in different birds, in its rela- 

 tive position to the pectoral ridge. In Baptores it is relatively 

 closer to the base of that ridge than in Argillornis. In Pelecanus 

 its relative position agrees better with that of the ancono-deltoid 

 ridge in the fossil, but it inclines distally toward the radial border 

 of the bone instead of from that border. In Diomedea both its 

 course and relative position (fig. 13, h) at the part of the base of the 

 pectoral ridge answering to that preserved in the fossil best agree 

 therewith ; but there is in Diomedea a second linear intermuscular 

 ridge (ib. Tc') anconad of the first, of which there is no trace in the 

 fossil. 



But the sum of my comparisons of the present portion of humerus 

 inclines me to see the nearest affinity to be to the longipennate 

 natatorial or aquatic birds, and among them to the largest existing 

 kind, viz. the albatross {Diomedea eaudans), but with a difference of 

 size indicated by the subjoined admeasurements : — 



Argillornis. Diomedea. 

 lines. lines. 



Transverse breadth of humerus at distal 1 



termination of the origin of the I 12 9 



pectoral ridge J 



A guiding modification of the distal portion of the humeral shaft 

 in birds is the difference of form of the radial and ulnar sides of the 

 bone. The radial side becomes, in most birds, the narrowest, as the 

 shaft descends and expands toward the distal articular end ; and in 

 the albatross it is carried to the extreme of becoming a mere ridge 

 for near 3 inches from that end (figs. 14, 15, 1). The portion of the 

 distal end of the shaft of the present fossil humerus, wanting the 

 articular termination of the bone (figs. 10-12), presents this character, 

 and, as in Diomedea, shows the narrow longitudinal groove on the 

 palmar side of the ridge (fig. 11). This groove is the radial border of 

 the triangular flat and shallow depression (figs. 11, 15, m) for the origin 

 of the " brachialis anticus " muscle, which depression shows the usual 

 linear roughness or sculpturing due to such relation of muscular 

 attachment. The same sculpturing marks the triangular " prebra- 

 chial depression" (fig. 11, m) in Argillornis; but the triangle is 

 longer and narrower in the fossil, and the free surface of the palmar 

 part of the shaft, ulnad of the depression, rises more abruptly from 

 it m Argillornis, and presents a more uniform transverse convexity 

 than in Diomedea. 



Unfortunately the characteristic ectepicondylar process (fig. 15, n) 

 of Diomedea has been broken away, if it ever existed in Argillornis, 

 with the rest of the distal end of the humerus in the present fossil 

 (fig. 11); but it may be remarked that the presence of such a projec- 

 ting part of the humerus, like the pectoral and subtuberous ridges 

 in Diomedea (figs. 4 & 5, c & i) would render the rolled fossil more 

 liable to such fracture and loss. 



In relation to other species and genera of birds based on fossil 



